1. Field of the Invention
This invention relates to radar systems. Specifically, the present invention relates to bistatic radar systems.
2. Description of the Related Art
In a mono-static radar system, the transmitter and the receiver are co-located. In a bistatic radar architecture, the transmitter and receiver are substantially separated. In addition, both the transmitter and the receiver may be mounted on either fixed or moving platforms. Bistatic radar is therefore distinguished from monostatic radar where the transmitter and receiver are mounted on the same platform and move together.
A characteristic feature of bistatic radar systems is that the transmitter, the receiver and the target, form an iso-range ellipsoid with the transmitter and receiver at the foci of the ellipsoid. In addition, the transmitter, the receiver and the target define the bistatic plane.
Further, the receiver, the target and the receiver""s motion relative to the target instantaneously define a plane, known as the xe2x80x9creceiver slant plane.xe2x80x9d Similarly, the transmitter the target and the transmitter""s motion relative to the target also define a plane, the xe2x80x9ctransmitter slant planexe2x80x9d, which is, in general, different from the receiver slant plane.
Any planar section through the ellipsoid is an ellipse. In particular, a plane tangent to the Earth""s surface cuts the ellipsoid in such a way as to produce an ellipse. Similarly, the bistatic plane, defined by the transmitter, the receiver and the target, cuts the ellipsoid in such a way as to create an ellipse.
In conventional airborne, as well as most ground based, bistatic systems the bistatic plane is nearly parallel with, and close to, the tangent plane of the Earth""s surface. In this circumstance, the ellipsoid approximately reduces to an ellipse which contains the bistatic plane and the velocity vectors of the transmitter, the receiver and the target. In this simplifying approximation, the receiver slant plane and the transmitter slant plane are practically coincident and both are practically coincident with the bistatic plane. It has therefore become a standard approximation of bistatic radar systems, that the motion of the transmitter and receiver lie within the bistatic plane. This approximate reduction of all system elements to a single plane greatly simplifies the analysis of bistatic radars. Unfortunately, the requirement that velocity vectors lie within the bistatic plane imposes significant constraints on the system and limits the operational flexibility.
A further limitation of conventional radar systems is that, in order to reach long ranges the beam must be narrow (i.e., the antenna gain must be high) so that the intensity of illumination falling on the target is sufficiently large for detection. This has led to a standard design approach whereby the narrowest illuminating and receiving beam is always considered the best.
In addition, when the ground is being illuminated from the air, long range observations result in a very shallow angle of illumination. The footprint of illumination is therefore spread out in a very long and narrow ellipse. For long range observation the parts of the ellipse that are near illuminate territory which is typically not of interest. Thus, much of the energy in the beam is wasted because it does not reach distant targets.
In some cases, such as air search radar, the beam is deliberately broadened so as to detect the presence of targets within a substantially larger volume of space. But the penalty for such broadening is a significant reduction in detection range. Thus, beam broadening is only occasionally pursuedxe2x80x94and then only for specialty radars.
One consequence of this conventional design philosophy is that only a very small slice of territory can be examined by the radar at any given time. In order to survey a large range of territory the narrow beam is usually swept through an arc. As a result of this beam sweeping technique, only a small fraction of the accessible territory will be observed at any given time. Events in the un-illuminated territory are unobservable.
Bistatic radar consists of a separate transmitter and receiver. In normal ground observing bistatic radars, both the transmitter""s illumination and the receiver""s direction of observation are usually at a very shallow angle to the surface of the Earth. The intersection of the two beams is usually a very small patch because the angle of intersection of the two beams is usually substantially large. If the target area of interest is small, this patch can be continuously observed and the signal to noise ratio of the observation can be satisfactory out to a substantial range. However, since in general, both the transmitter and receiver are moving with respect to the target, special coordination between the illumination beam, receiver observation beams and the directions to the target must take place. This introduces a beam coordination problem known as the Scan-On-Scan beam coordination problem.
In the Scan-On-Scan operational mode a conventional bistatic radar illuminates a small region with a very narrow beam. When the transmitter beam moves, the receiver beam must move in a coordinated way to track the transmitter beam and follow a single target or small patch of territory. Alternatively, with Scan-On-Scan operation, the receiver beam can be fixed. In this case only a small area of territory is observed during the transmitter scan. Similarly, when the receiver beam scans, only a small area of territory is observed during each instant of the receiver scan.
FIG. 1A displays a Scan-on-Scan operation, including a transmitter and receiver. FIG. 1A highlights the transmitter scan operation. FIG. 1B displays a Scan-on-Scan operation, including the transmitter and receiver shown in FIG. 1A. FIG. 1B highlights the receiver scan operation. Both FIG. 1A and FIG. 1B display a transmitter beam overlapping a receiver beam (e.g., item 120). In FIG. 1A a transmitter 100 generates a narrow transmitter beam in a first position 102 and then scans through an angle depicted by 104 to a second position 106. A receiver 108 is also shown generating a very narrow beam 118. The transmitter 100 and the receiver 108 are also shown in FIG. 1B. In FIG. 1B the receiver 108 generates a narrow beam 110 in a first position and scans through an angle depicted by 112 to a second position shown by 114. During the respective scanning operations, the beam from the transmitter 116, overlaps with the beam from the receiver 118, in a very narrow overlapping region 120 as shown in both FIGS. 1A and 1B. The very narrow overlapping region 120, is the observable target region of the system.
As shown in FIGS. 1A and 1B, if a bistatic radar is to be used to observe a large territory, the transmitter beam and the receiver beam must be separately scanned across the landscape. During these scans only a small fraction of the illuminating energy will find its way to the receiver at any given time. This means that for broad area observation, bistatic radars tend to be very energy inefficient.
When either the transmitter or the receiver is in motion, a bistatic radar system can create a high resolution two dimensional image of the landscape. (If the target is moving, but the transmitter and receiver remain stationary, a high resolution image of the target can similarly be constructed.) With motion of either the transmitter or the receiver the reflected signal will be Doppler frequency shifted as a function of relative motions and positions of the transmitter and receiver and the position of the target. In effect, the motions and positions of the transmitter and receiver paint the landscape with a spatial Doppler frequency gradient. If the transmitter and receiver are both moving in a similar direction the Doppler gradients add (in a vector sense) thereby creating a stronger Doppler gradient at the target. From the Doppler shift produced by this Doppler gradient it is possible to derive the azimuthal position of a given target object in the landscape. When this frequency gradient derived azimuth information, is combined with range information, which is derived from processing the radar""s pulses, a high spatial resolution two-dimensional picture of the landscape can be formed. The technique creates a bistatic Synthetic Aperture Radar (SAR) which is closely akin to the widely used monostatic SAR.
While monostatic SAR radar systems are effective at producing high resolution images of a terrain, it is often difficult for them to extract images of moving targets located within the terrain. This is particularly true because the signals from moving targets spread out and therefore reduce in intensity relative to the terrain reflections. Such terrain dominant imagery is typically known as xe2x80x9cimage clutterxe2x80x9d (e.g., clutter Doppler spectrum).
Bistatic radar provides an interesting and useful solution to the image clutter problem. When the motion of the receiver is in the opposite direction to that of the transmitter the Doppler spatial gradients cancel. This cancellation condenses the clutter Doppler spectrum into a narrowed frequency spectrum. The gradient cancellation is conventionally known as xe2x80x9cclutter condensation.xe2x80x9d Characteristically, clutter condensation narrows the spectrum of the clutter but the Doppler offset of a moving target remains the same. With clutter condensation the returns from relatively slowly moving targets may therefore be found outside the clutter spectrum and may become highly observable. Thus, by suitably opposed motions of the transmitter and receiver, a bistatic radar system can become much more effective at detecting slowly moving targets than an equivalent monostatic radar wherein slow moving targets are lost in clutter.
In a typical conventional bistatic radar system clutter condensation operation, the transmitter aircraft and the receiver aircraft circle the target so that the two aircraft are always on opposite sides of the circle and the target is in the middle. In addition, both aircraft have to proceed in the same angular direction at the same angular velocity (for example, both are moving clockwise). Once this constrained geometry is maintained then clutter cancellation will take place in a small region around the target.
In conventional bistatic radar, the bistatic plane is (nearly) parallel to the Earth""s surface. In addition, the transmitter and receiver motions effectively lie within the bistatic plan. These geometrical constraints significantly limit the conditions where clutter condensation is effective in isolating slowly moving targets.
Thus, there is a need in the art for a bistatic radar system which is capable of greater operational flexibility. In addition, there is a need for a radar system capable of discriminating targets in the presence of image clutter.
The need in art is addressed by the bistatic radar architecture of the present invention. In an illustrative embodiment, a bistatic radar system is disclosed in which a bistatic radar receiver is located in a first plane and a transmitter is located in a second plane. The first plane, or receiver slant plane, is different from the second plane or transmitter slant plane. The transmitter is located in Middle Earth Orbit (MEO). The transmitter plane is located at a position that is substantially tilted toward the vertical with respect to the first plane. As a result of this configuration, greater operational flexibility to fly various paths and trajectories is provided, which facilitates enhanced detection of targets.
In an illustrative embodiment, the instantaneous illumination from the transmitter is spread over a relatively large area of the Earth compared with the area of instantaneous illumination from conventional scanning radar systems. Because the illuminated region is broad the transmitting antenna can be physically small. For a transmitter onboard a MEO satellite, the transmitting antenna need only be a few meters in diameter. This relatively compact antenna results in substantial cost savings compared with conventional radar satellite designs.
A major benefit of instantaneously illuminating a substantially large area of the Earth is that many different receivers can simultaneously observe reflected signals from different parts of the illuminated region. These observations can be made independently or, they can be coordinated, either incoherently or coherently. Coherent observation is possible because all the participating receivers see the same coherent illumination. Thus, receivers can be connected together to make physically very large phased antenna arrays.
In the present invention, several embodiments are disclosed. In each embodiment, the transmitter and receiver are normally located in different slant planes, although in some cases the receiver and transmitter slant planes may be coincident. In one embodiment, the transmitter moves in a first direction and the receiver moves in a second direction which is opposite from (that is, anti-parallel to) the first direction. In a second embodiment, the transmitter moves in a first direction and the receiver moves in the same direction (that is, the transmitter and the receiver move parallel to each other). In a third embodiment, the transmitter moves in a first direction and the receiver moves in a second direction, which is neither parallel to, nor anti-parallel to the first direction. In a fourth embodiment, the transmitter moves in a first direction and the receiver is stationary. In a fifth embodiment, the transmitter sweeps an illuminated area with a narrow beam.
In the method and apparatus of the present invention a bistatic radar systems clutter condensation solution is presented, in which a transmitter aircraft and a receiver aircraft circle a target, on the same side of the target. Their motion is opposedxe2x80x94but not necessarily circular. This geometry enables practical clutter cancellation over broad sweeps of territory.